Zooming and loading very large TIFF file

问题:

I have a very large hi-res map which I want to use in an application (imagesize is around 80 mb).

I would like to know the following:

• How can I load this image the best way possible? I know it will take some seconds to load the image (which is ok) but I would like to notify the user of the progress. I would like to use a determined mode and show this in some sort of JProgressBar to the user. This should reflect the number of bytes that have been loaded or something like that. Is there any Image loading method that can provide this functionality (like ImageIO.read())?
• Because the map is of very high resolution I would like to offer the user to scroll to zoom in and out. How can I do this the best way? I know for a fact that rescaling a BufferedImage the standard way would take a VERY long time for such a big file. Is there any efficient way of doing this?

Thank you for your input!

kind regards,
Héctor van den Boorn

p.s. The image will be drawn on the canvas of a JPanel.

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Hi Andrew, Thank you so much for your help; everything worked out perfectly and is loading quick. Without your expertise and explanation I would have still been working on this so you've earned the bounty fair and square.

What I did was the following; using the imagemagick I created multiple images of different resolution and at the start of execution I load only the smallest res. image. The rest are loaded in seperate threads so execution is not stalled. Using the information you provided me I then use the appropriate images when zooming in or out. I'm a bit sceptical of using the tiles because I need to draw my own images on top of the map and I couldn't find the paint function in the external jar you told me to use, so I ended up using something simple; when zooming or panning the rescale mode is set to fast and when you're not zooming or panning the rescale is set to smooth for pixel-perfect images (just like you suggested), but this turns out to be fast enough and I don't need tiles (altough I do see that with even larger images this would be necesarry and I understand the information you've given me).

So thanks again and everything is working perfectly :)

回答1:

There are two approaches you should (simultaneously) take:

1. Downscaling your image into various sizes. You should downscale your image at a series of lower resolutions (1/2, 1/4, 1/8, etc until the image is about the largest likely screen resolution). When the user first opens the image, you show the lower resolution image. This will load fast and allow the user to pan. When the user zooms in, you use a higher resolution image. You can use ImageMagick for this: http://www.imagemagick.org/Usage/resize/
2. Tile your larger images. This breaks down the single, large image into a large number of small images in a grid pattern. When a user zooms in on an area, you compute which tiles the user is looking at, and you render only them, not the other areas of the image. You can use ImageMagick to do split an image into tile, eg ImageMagick. What is the correct way to dice an image into sub-tiles. The documentation is http://www.imagemagick.org/Usage/crop/#crop_tile

(Providing a cache of appropriately sized and tiles images is what allows GoogleEarth and countless other mapping applications, to render so fast, yet zoom into the map at incredibly high resolution)

Once you have your tiles, you can use one of several engines in Java:

• https://wiki.openstreetmap.org/wiki/Tirex
• http://www.slick2d.org/wiki/index.php/Tiled

There may be others as well.

You can implement arbitrary zooming (suitable for pinch-to-zoom or similar) within this framework. Within the zoom limits you allow, your algorithm would be something like:

1. For the zoom level chosen by the user, choose the closest higher resolution cache. For example, if you have 100%, 50%, 25% and 12.5% tiles, and the user chooses 33% zoom, select the 50% tiles
2. Set the layout for the tiles so the tile squares have the correct size for the chosen zoom (this might be a single tile at lowest zoom levels). For example, at 33% zoom using 50% tiles, with the tiles being 100 pixels square, the grid will be 67 pixel squares
3. Individually load and scale the tile images to fit the screen (this can be multi-threaded which works well on modern CPU architectures)

There are a couple of points to note:

1. The scaling algorithm changes when you reach the greatest resolution you have tiles for.
   1. Up to 100% zooming for the image, use bilinear or bicubic scaling. This provides excellent appearance for photographs with little jaggedness
   2. Above 100%, you probably want to show the pixels, so nearest-neighbour might be a good choice
2. For higher fidelity, use a higher scale tile and downscale > 50%. For example, suppose you have tiles prepared at 100%, 50%, 25% and 12.5%. To show 40% zoom, don't scale down the 50% tiles; instead use the 100% tiles and scale them down to 40%. This is useful:
   1. If your images are textual or diagrams (i.e. the raster images containing many straight lines). Scaling these type of images will often produce nasty artefacts if you don't oversample
   2. If you need very high fidelity on photographic-style images
3. If you need to render a preview of the zoom (eg while the user is still pinching-and-zooming), grab a screenshot at the start of the gesture and zoom that. It matters much more that the animation is smooth than the zoom preview is pixel-perfect.
4. Selection of the right size of tile is important. Very large tiles (<1 per screen) is slow to render. Too small tiles creates other overheads and often produces nasty rendering artefacts where you see the screen filling up randomly. A good compromise between performance and complexity is to make the tiles about a quarter of the full-screen size.

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When using these techniques, the images should load very much faster and so the progress bar is not so important. If it is, then you need to register a IIOReadProgressListener on the ImageReader:

• ImageReader.addIIOReadProgressListener()

From the JavaDoc:

An interface used by ImageReader implementations to notify callers of their image and thumbnail reading methods of progress.

This interface receives general indications of decoding progress (via the imageProgress and thumbnailProgress methods), and events indicating when an entire image has been updated (via the imageStarted, imageComplete, thumbnailStarted and thumbnailComplete methods). Applications that wish to be informed of pixel updates as they happen (for example, during progressive decoding), should provide an IIOReadUpdateListener.